This is a theory question. I want to know the difference between a Single Nucleotide Polymorphism and a Polymorphism. They are not exactly the same thing but I can't understand the difference. SNP is a single nucleotide change that occurs in a population with a frequency >=1% (by the conventional definition). Now, a polymorphism isn't also a change that can occur in a population?
It depends on the particular field. Different groups use slightly different definitions of SNPs.
For example, I've seen SNP where S stands "small", not "single", which would include indels. Also, a SNP may be considered a SNP regardless of its population frequency as long as it's germline.
I'd think that every SNP is a polymorphism, but not all polymorphisms are SNPs, as they can be indels or CNV (copy number variants/variation), for example. The word 'polymorphism' comes from the Greek: poly: several; morphe: forms, states. I'm not sure if the word polymorphism is standard in the Sequence Ontology (SO) project. You can search for it, but you will get redirected to SNV (single nucleotide variants) for example rather than SNP. The latter is rather well established in the literature and scientific usage but SO and others may try to avoid using that term. For example, Ensembl avoids using the word polymophism and use the word variant instead. I think in the past polymophism was used to differentiate from mutation, the latter would cause a disease but not the former.